Speckle patterns are irregular, fine-grain light distributions which may occur when illuminating white walls, projection screens and other surfaces (hereinafter largely referred to as, for example, projection screens) using widened, coherent light, in particular laser light. The speckle pattern may be formed when a spot of light is imaged on the projection screen, due to the high coherence caused by interference of the light waves scattered at various points on the projection screen. The interference pattern may exhibit the stochastic fine structure of the reflecting screen. The average size of a speckle grain depends on the aperture of the coherently illuminated spot on the screen. The larger the light spot is, the finer is the graininess of the speckle pattern. The contrast in the speckle grains may be determined by the coherence of the light source. The speckle pattern may disappear when the coherence length of the light falls perceptibly below the average roughness of the screen.
It is believed that to optically reproduce images, laser-based projection methods are being used to an increasing degree. In contrast to image rendition using cathode-ray tubes or liquid-crystal displays, the laser projection technique may fundamentally enable a high-quality image to be attained with an unlimited image size. In this context, the laser beam for displaying the image to be rendered may be rasterized similarly to an electron beam in a picture tube via a projection screen.
The speckle formation encountered in projection methods using lasers or other coherent light sources is disadvantageous. Speckles can occur, in particular, when the image is built up from individual image points, line-by-line, and, to this end, laser beams are focused on the projection screen. Due to the small image points, the speckle pattern may be coarse-grained and perceived by the observer as a disturbing glittering of the individual image points.
It is believed that there are various basic approaches for suppressing speckle formation. The reference “Speckle Reduction in Coherent Information Processing”, Toshiaki Iwai and Toshimitsu Asakura, Proceedings of the IEEE, vol. 84, no. 5, May 1996, pp. 765-780, may provide an overview of some basic approaches. The methods described can be broken down into methods for controlling spatial coherence, controlling temporal coherence, each implemented by manipulating the light source, spatial scanning, spatial averaging, and speckle reduction through digital image processing.
A pulsed laser light source having a small pulse length may be used, thereby reducing the coherence length of the laser light and minimizing speckle formation. However, this may only permit the use of laser systems, which are able to be simply modulated, externally or internally. Furthermore, the spatial coherence of the laser light can be reduced by passing the laser light through a rotating ground glass screen or by scattering it at one or a plurality of optical diffusers. Coherent laser light may be coupled into a multimode optical fiber and the fiber may be deformed by subjecting it to rotation or vibration. At the end of the fiber, the light may emerge, having been separated into a multiplicity of modes in the local space, each mode having traversed a different optical path and, therefore, having different phase positions. By vibrating or rotating the fiber, the mode distribution may be varied over time. Thus, a temporal and spatial average may be generated over the phase pattern being formed, and an incoherent, even if multimode, light source may be provided. This mechanical approach for thoroughly mixing the modes can adversely affect the stability of the overall arrangement.
To reduce the temporal coherence of the laser light, the wavelength of the laser light may be varied or a plurality of wavelengths may be used at the same time. For example, to reduce speckles, a method based on a change in the wavelengths of laser diodes caused by mode jumps may be used. Other lasers as well, which are subject to random fluctuations, come into consideration for this.
An alternative approach for reducing speckle formation is purportedly discussed in the reference German Patent No. 196 45 976. That reference discusses using a projection screen, whose projection depth is greater than the coherence length, so that the reflected or transmitted wave field becomes incoherent. This can entail that the image points are diffusely enlarged by the surface structure of the screen, as well as the limitation of always having to use a specially prepared screen for image rendition.
The reference PCT Patent Publication No. WO 96/21883 purportedly discusses a projection screen whose surface is formed in irregular fashion, inter alia, for purposes of reducing speckles, such that the Fourier spectrum of the surface exhibits higher frequencies than that of a pixel structure projected onto the screen.
The reference German Patent No. 195 08 754 purportedly discusses a method for reducing the interference of a coherent light beam, the light being polarized variably with respect to location, in a direction perpendicular to the direction of propagation. In this case, the circumstance is utilized that different polarization states of the light are no longer able to completely interfere with one another. It is believed that the required polarization states can be produced, for example, with the assistance of LCD matrices.
The reference German Patent No. 107 10 660 purportedly discusses a device for removing screen speckles when working with scanning laser-image projection, the laser beam being split with the assistance of an ultrasound cell in which density waves travel, by the diffraction of the density waves into various orders of diffraction of different frequencies. The beam components are superposed using a lens. In this manner, a moving system of interference patterns is apparently formed on the projection screen, so that the forming speckles overlap one another in the eye of the observer due to the integration process, and become averaged out in time and space.
The reference U.S. Pat. No. 3,941,456 A purportedly discusses a device for reducing granulation, which occurs when transmitting optical information using a high-grade, coherent light beam. The light beam propagates through an ultrasound cell in which, depending on the excitation, standing or traveling density waves may form. The density waves may influence the refractive index locally, so that the light beam propagates through zones having different refractive indices, resulting in a reduction in the granulation.
The reference “Perceived Speckle Reduction in Projection Display Systems”, IBM Technical Disclosure Bulletin, U.S.A., IBM Corp., New York, vol. 40, no. 7, Jul. 1, 1997, pages 9-11, XP 000728388, ISSN 0018-8689, purportedly discusses reducing speckles in that the light beam propagates through a liquid crystal, whose refractive index is influenced by an electrical field.
The reference U.S. Pat. No. 4,647,158 purportedly discusses a method and a device used to convert a beam of coherent light using a controllable diffraction grating into a beam of incoherent light.